He dismissed Trump's claim the U.S. can slap tariffs on steel and aluminum for national security reasons.

Juncker said that "none of this is reasonable, but reason is a sentiment that's very unevenly distributed in the world."

Asked if a trade war is brewing, he said: "I can't see how this isn't part of war-like behavior."

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12 p.m.

The head of the World Trade Organization says his agency is "clearly concerned" about President Donald Trump's plan to impose tariffs on imported steel and aluminum and is warning of the potential for escalation.

Roberto Azevedo says a "trade war is in no one's interests."

A WTO spokesman relayed Azevedo comments to The Associated Press in an email just hours after President Donald Trump tweeted: "When a country (USA) is losing many billions of dollars on trade with virtually every country it does business with, trade wars are good, and easy to win."

Azevedo warned: "The potential for escalation is real, as we have seen from the initial responses of others."

He said the WTO "will be watching the situation very closely."

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9:35 a.m.

Sen. Ben Sasse is attacking President Donald Trump over his comments that trade wars are "easy to win."

The Nebraska Republican lashed out at Trump's defense of placing new tariffs on steel and aluminum imports. In a statement, Sasse says: "Trade wars are never won. Trade wars are lost by both sides."

Sasse continues: "Kooky 18th century protectionism will jack up prices on American families -- and will prompt retaliation from other countries. Make no mistake: If the President goes through with this, it will kill American jobs -- that's what every trade war ultimately does. So much losing."

Trump declared Thursday that the U.S. will impose steep tariffs on steel and aluminum imports. The announcement escalated tensions with trading partners and stocks closed sharply lower on Wall Street.

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8:30 a.m.

A White House spokeswoman says President Donald Trump is not concerned about the stock market decline following his announcement of new aluminum and steel tariffs.

Sarah Huckabee Sanders said Friday that the "president is still focused on long term economic fundamentals." She called the steel and aluminum industries "the backbones of this country."

Trump's decision to impose punishing tariffs on steel and aluminum imports has escalated tensions with China and other trading partners and raised the prospect of higher prices for American consumers and companies.

His announcement came only after an intense internal White House debate and it brought harsh criticism from some Republicans.

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6:10 a.m.

President Donald Trump is arguing for steep tariffs on steel and aluminum imports, saying "trade wars are good."

In a tweet Friday, Trump writes: "When a country (USA) is losing many billions of dollars on trade with virtually every country it does business with, trade wars are good, and easy to win."

He argues that the U.S. is "down $100 billion with a certain country and they get cute, don't trade anymore-we win big. It's easy!"

On Thursday, Trump said firm action was crucial to protect U.S. industry from unfair competition and to bolster national security. However, his announcement came only after an intense internal White House debate. It brought harsh criticism from some Republicans and roiled financial markets with concerns.

Families of mass shooting victims are joining California State Treasurer John Chiang in calling on the nation's largest public pension fund to stop investing in companies that sell assault weapons and devices that allow guns to fire more rapidly.

A Trump-affiliated firm under scrutiny for inappropriately obtaining data on tens of millions of Facebook users created profiling algorithms that "took fake news to the next level," a former employee said.